linux-kernel - Re: [RFC PATCH 3/3] sched/fair: Add a per-shard overload flag

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Date:   Thu, 28 Sep 2023 16:41:10 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     K Prateek Nayak <kprateek.nayak@....com>
CC:     David Vernet <void@...ifault.com>, <linux-kernel@...r.kernel.org>,
        <peterz@...radead.org>, <mingo@...hat.com>,
        <juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
        <dietmar.eggemann@....com>, <rostedt@...dmis.org>,
        <bsegall@...gle.com>, <mgorman@...e.de>, <bristot@...hat.com>,
        <vschneid@...hat.com>, <tj@...nel.org>, <roman.gushchin@...ux.dev>,
        <gautham.shenoy@....com>, <aaron.lu@...el.com>,
        <wuyun.abel@...edance.com>, <kernel-team@...a.com>
Subject: Re: [RFC PATCH 3/3] sched/fair: Add a per-shard overload flag

On 2023-09-27 at 14:06:41 +0530, K Prateek Nayak wrote:
> Hello Chenyu,
> 
> On 9/27/2023 12:29 PM, Chen Yu wrote:
> > Hi Prateek,
> > 
> > On 2023-09-27 at 09:53:13 +0530, K Prateek Nayak wrote:
> >> Hello David,
> >>
> >> Some more test results (although this might be slightly irrelevant with
> >> next version around the corner)
> >>
> >> On 9/1/2023 12:41 AM, David Vernet wrote:
> >>> On Thu, Aug 31, 2023 at 04:15:08PM +0530, K Prateek Nayak wrote:
> >>>

[snip]

> > This makes me wonder if we can let shared_runq skip the C/S tasks.
> > The question would be how to define C/S tasks. At first thought:
> > A only wakes up B, and B only wakes up A, then they could be regarded as a pair
> > of C/S
> >  (A->last_wakee == B && B->last_wakee == A &&
> >   A->wakee_flips <= 1 && B->wakee_flips <= 1)
> > But for netperf/tbench, this does not apply, because netperf client leverages kernel
> > thread(workqueue) to wake up the netserver, that is A wakes up kthread T, then T
> > wakes up B. Unless we have a chain, we can not detect this wakeup behavior.
> 
> Yup, unless we have a notion of chain/flow, or until we can somehow
> account the wakeup of client using the kthread to the server, this will
> be hard to detect.
> 
> I can give it a try with the SIS_PAIR condition you shared above. Let
> me know.

Thanks Krateek, but I don't think SIS_PAIR could bring benefit to the netperf/tbench
since SIS_PAIR can not detect the chain wakeup.

thanks,
Chenyu